A group of international mediators led by the former UN secretary general Kofi Annan formally called on the armed Basque group Eta to lay down arms on Monday, in a key step in ending decades of terrorism in Spain.

The appeal made by a group that includes Sinn Féin's Gerry Adams and the former Irish prime minister Bertie Ahern is expected to bring a positive response from Eta, though it remains unclear exactly how far the much-weakened separatist group is ready to go.

Sources close to negotiators believe Eta will produce a statement within days that will effectively mark the end of more than four decades of violence that have caused some 830 deaths.

"We believe it is time to end, and it is possible to end, the last armed confrontation in Europe," Annan's group said.

"We call upon Eta to make a public declaration of the definitive cessation of all armed action," the group added in a statement read out by Ahern in the Basque city of San Sebastian, northern Spain.

It urged Eta and the French and Spanish governments to enter talks "exclusively to deal with the consequences of the conflict".

Representatives of political parties close to Eta are expected to echo the call on Tuesday, putting further pressure on it to stick to a peace script written with the help of Annan's group.

Eta is not expected to dissolve, but is in a poor bargaining position after years in which police action has left it close to defeat.

"What Eta ought to talk about now are consequences, not causes," said Iñigo Urkullu, leader of the moderate Basque Nationalist party.

Politicians are prepared to make few concessions beyond negotiating the future of Eta members in jail. Some commentators have called on Annan's group to stay out of the Basque country.

"If they think there has been a war here, they are wrong," said Rúben Múgica, the son of an Eta victim. "All there has been is totalitarian persecution."